Mobile computing devices, for example cellphones, tablets, laptops of various mechanical configurations and similar electronic user apparatus comprise audio transducers, including but not limited to microphones or loudspeakers for communicating, recording, playing back or otherwise processing audio signals such as voice or music. Increasingly, such apparatus may comprise multiple microphones or speakers to allow more complex audio signal processing, which may be implemented on a device such as a smart codec or other integrated circuitry comprising digital signal processing capability.
By way of example in accordance with the prior art, FIG. 1 illustrates a cellphone 100 comprising six audio transducer devices: four digital microphones 110, 111, 112, 113 and two digital loudspeakers 120, 121. Each of these transducer devices needs to receive or transmit streams of audio data from or to a smart codec 130. The microphones in particular are very small, of the order of 1 mm on a side, so can only accommodate a very limited number of electrical terminals or pins. The codec and the digital loudspeaker may also have a limited number of pins. Thus, the audio data may be transmitted by serial buses carrying the required clock and data information on only one or two lines, e.g. wires. One example of such a serial interface is the commonplace multiplexed pulse-density-modulated (PDM) format, where a “left” microphone and a “right” microphone transmit pulse-density-modulated (PDM) or delta-sigma oversampled data streams in alternate half-cycles of a clock as illustrated in prior art FIG. 2a. This format allows two channels of audio data to share a single bus of two wires (one carrying clock to the microphone, one carrying the multiplexed data back from the microphones in a time-multiplexed fashion). Data can be transmitted to pairs of loudspeaker devices in a similar fashion. Thus cellphone 100 comprises three TDM audio buses 131, 132, 133 as illustrated in FIG. 1.
Rather than separately manufacture “left” (L) and “right” (R) channel microphones, designed to output respective PDM audio data in first or second clock half-cycles, digital microphones conventionally comprise one further pin, and are configured to output data bits in the first half clock cycle if this pin is tied to a logic high voltage, typically the microphone positive supply voltage, and to output data bits in the second half clock cycle if this pin is tied to a logic low voltage, typically ground. In other words, the same digital microphone may contribute either the L or R components of the composite DATA waveform shown in FIG. 2a. 
However, such an interface format only allows two audio data channels to share a bus, and (except possibly for test modes) only to carry data in one direction. There are other more complex serial data interface formats, for example Soundwire™ interface format, which allow multiple channels of audio data, say up to 16 channels, in either direction, and also to convey control data in both directions. Such formats may contain framing information, but during the periods when transmitting payload data, the waveforms on the bus may similarly comprise half-clock-period durations in which each device on the bus may send data in turn, as illustrated in prior art FIG. 2b. 
In some applications, the simpler scheme is more appropriate, for example for cheaper and less flexible systems, whereas in more feature-rich applications, the more flexible but more complex bus and related interface circuitry is appropriate. Also, similar models of device may have less or more functionality.
Rather than having separate models of microphone device for respective different buses, it is advantageous to have a device designed so as to be capable of accommodating operation in either mode. Thus, only the one device needs to be qualified for production of the end-user apparatus, even for different models. Also, as models evolve, for example suitable Soundwire codec circuits become commercially available to replace the incumbent PDM codec, the Soundwire codec can be adopted without having to re-engineer the acoustics or re-qualify the system as would be required if a different model of microphone were required.
However, to be able to operate in either mode requires such a device to be able to recognize what sort of interface it is connected to. Attempts to decode this from the present bit patterns on the two wires have proven not be to adequately reliable. It is undesirable to add another pin onto the digital microphone as this would either increase the size of the microphone device package or reduce the dimensions of each pin or pad below that desirable for reliable connection onto the underlying circuit board.
For buses, such as Soundwire, which carry more than two channels of information, each device attached to the bus requires an address or similar identity information. If there are more than two devices attached to each bus, then the simple “left/right” pin strapping technique above can no longer identify each device. Especially for microphones, it is undesirable to have multiple identity pins each tied high or low to define the binary address.
Thus, there is a desire for a low pin-count method of identifying a device in an end-user apparatus and to define whether the device is to communicate via say a PDM format or some other format such as Soundwire.
Similarly, digital loudspeaker devices also require the ability to select an interface format and indicate the device's identity to the bus.